The present invention relates to measuring the response time (i.e. either the attack or the release time) of acoustical devices in general; and more particularly, it relates to a method and apparatus for automatically measuring the time period of transient response in an electronic circuit having feedback control to maintain its output amplitude within desired limits without clipping. Thus, the system may be used to measure response times for hearing aids, earphones, automatic gain or volume control circuits or compression circuits.
By way of example only, the attack time and release time of a hearing aid having automatic gain control are well-defined performance parameters of hearing aids; and specifications have been established in the industry for measuring attack time and release time. Briefly, a tone signal of constant frequency (for example, 2000 Hz) and having an acoustical power level of 55 dB Sound Pressure Level (SPL) is increased to 80 dB SPL; and the attack time is defined as the amount of time it takes the hearing aid to return to and stay within a range of .+-.2 dB of its quiescent or "steady state" response at the 80 dB SPL input level.
In measuring the release time, the input signal is abruptly changed from the 80 dB SPL level down to the 55 dB SPL level; and the release time is defined as the amount of time it takes the hearing aid to maintain a value within .+-.2 dB of its steady state response at the 55 dB SPL input level. As indicated above, these measurements are well defined in the industry, and the attack and release times are accepted as performance criteria for hearing aids. The present invention has much broader application, however, as persons skilled in the art will appreciate; and, for example, any one or more of the above levels, times, or frequencies can easily be modified without limiting the invention.
As used herein the input acoustical signal, whether at the 55 or 80 dB SPL levels (or other level) is referred to as the "test tone". The output signal level of the device under test in response to an input test tone and after all transient responses have subsided is referred to as the "quiescent" or "steady state" response of the device. The .+-.2 dB range above and below the steady state response within which the output level of the unit must settle in measuring recovery times (whether attack or release) is referred to as the "recovery range". The upper limit (steady state response plus 2 dB) of the recovery range is called the "upper threshold"; and the lower limit (steady state response minus 2 dB) is called the "lower threshold".
Currently, attack and release times of hearing aids are usually measured manually. This method is time consuming and subject to the error of subjective measurement by a skilled operator. Further, it requires the use of expensive equipment, such as an oscilloscope with storage capability or an oscilloscope capable of taking a photograph of the time-varying response. Typically, after the scope controls are set up by the operator and adjusted to obtain the optimum sensitivity and time base display scales, and to insure proper triggering of the scope, measurements of the steady state response representing the output of the hearing aid to be tested are made at first and second levels. Voltages corresponding to .+-.2 dB of the steady state levels then have to be calculated from the measurements; and the hearing aid is subjected to a second input signal which changes from one of the levels to the other during which the actual time measurements for attack and release for the calculated steady state levels is made.
These measurements are made visually on the oscilloscope response, based upon the .+-.2 dB response levels calculated for the first test measurement. It normally takes two or three attempts before obtaining proper settings of the oscilloscope display for obtaining an accurate measurement. The response level normally changes from device to device, so the complete set up and initial calculation of recovery range may have to be made for each device. Obviously, the time required of a skilled operator to perform this test may be substantial.
Further, it may be desirable to measure attack and release times for different input signal levels; and to do this, the oscilloscope sensitivity ranges have to be changed. For low level signals, the oscilloscope may not be sensitive enough to provide a sufficiently large display to permit good resolution. This may result in reduced accuracy of measurement.
The measurement of attack and release times through the visual observation of an operator may be further complicated if the response of the unit being tested oscillates through the calculated recovery range. For example, the unit may exhibit a damped oscillation or "ringing" effect. In the case of measuring release time, the envelope of the response may undershoot the recovery range and even overshoot it thereafter. The operator must then determine which of the peaks and valleys of the oscillating response envelope are to be taken for measurement purposes.
The present invention is designed to perform attack and release time measurements for hearing aids with automatic gain control without the intervention of an operator and without requiring the operator to have any skills whatever in interpreting specific responses. In accordance with the present invention, the desired measurement is displayed in numeric form on a digital display, thereby obviating the need for interpretive measurement.
According to the present invention, a test tone signal of constant frequency is successively cycled between a relatively low power level and a relatively high power level. According to standards for measuring attack and release times of hearing aids with automatic gain control now in effect, these levels are 55 and 80 dB SPL respectively. Thus, the test tone signal generator generates a continuous wave, sinusoidal acoustical signal of 55 dB SPL for two seconds, then abruptly changes the power level to 80 dB SPL for 0.5 seconds, followed by another two second interval of tone at the 55 dB SPL level and finally again increases the level to 80 dB SPL for 0.5 seconds. This signal is coupled through a loudspeaker in a sound chamber in which the hearing aid unit under test is also enclosed. The operator sets a function switch to either the attack time measuring mode or the release time measuring mode. The test tone may be a constant frequency of 2,000 Hz.
Assuming that the release time is being measured, the operator sets the function switch accordingly, and during the first burst of test tone at the 55 dB level, the gain of an input amplifier coupled to the hearing aid is automatically adjusted so that the detection circuitry is operating with the input amplifier in a linear range. After the gain of the input amplifier has been adjusted and the response of the test unit has settled to a quiescent value, and still during the first burst of test tone at the 55 dB level, the response of the unit under test is measured by a peak detector circuit, and the response is stored in a sample and hold circuit. The output of the sample and hold circuit is used as a reference signal and coupled to a pair of comparators. One of the comparators is set to detect at the upper limit of the recovery range, and the other comparator detects the lower limit of the recovery range. The input signal is also coupled to the signal inputs of the comparators.
After the sample and hold circuit has stored a signal representative of the quiescent response of the unit under test at the 55 dB level, the test tone signal is increased to the 80 dB level; and again, the unit under test is permitted to achieve quiescent operation at the higher level. Following that, the test tone signal abruptly reduces to the 55 dB level, and this commences measurement for the release time by enabling a clock pulse train of predetermined repetition rate (1 millisecond for example) to be generated. The clock pulses are fed to a counter, including three individual BCD counters representative respectively of the units, tens and hundreds positions (in milliseconds) of the measurement. The clock pulses are accumulated in the counter. The outputs of the three counters are fed respectively to three registers, the outputs of which are coupled to digital displays for the three numerical positions of readout. During the measurement, the counters continue to accumulate clock pulses, and the comparator circuits enable the registers to be updated each time the response signal makes an excursion outside of the predetermined recovery range.
As an example, in the case of measuring release time at the start of a measurement when the sound pressure level is decreased, the measured signal will undershoot the lower threshold level of the recovery range. When the response level goes below the lower threshold level, the absence of output from the lower limit comparator permits a missing pulse detector to time out; and the output of the missing pulse detector causes the registers to be updated with current information contained in the counters. The missing pulse detector prevents false updating of the registers by noise. Whenever the response signal exceeds the upper threshold level of the recovery range, each individual tone signal is used to update the registers. Thus, when the response signal settles within the recovery range, the last updated accumulative count stored in the registers is a measure of the time in which it took the response signal to settle within the recovery range, and this becomes the release time for the unit under test.
The circuitry operates in a similar manner for measuring attack time, except that the sample representative of the steady state response is taken during the first burst of test tone signal at the 80 dB level after a quiescent value has been reached; and measurement commences the second time the test tone signal is increased from 55 dB to 80 dB.
With the present invention, a measurement of release time appears within about three seconds after the measurement is initiated by the operator; and an attack time measurement appears within about five seconds after the instrument is placed in the attack measuring mode and the measurement initiated. Further, there is no visual interpretation required on the part of the operator--rather, a simple, easily recorded digital measurement is visually presented to him. The present invention thus eliminates tedious, interpretive measurements, it shortens the time required for measurements, and it increases the accuracy of measurement. The present invention also has high immunity to a burst of acoustic noise.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.